The present invention relates generally to an automatic transmission comprising a plurality of power transmission paths for forward driving and a power transmission path for rearward driving, which paths are selected by actuation of hydraulic clutches that establish speed change ratios in automatic speed change.
Such automatic transmissions have been known and are utilized, for example, as automatic transmissions for vehicles. Typically, an automatic transmission designed for use in a vehicle operates such that the transmission automatically controls the actuation of hydraulic clutches to change the speed change ratio in correspondence to the driving condition of the vehicle. Generally, the automatic transmission includes a speed change hydraulic unit which comprises a plurality of shift control valves, a solenoid valve to control the actuation of these shift control valves, and a manual valve operated in correspondence to the movement of the shift lever manipulated by a driver. In this arrangement, the automatic transmission automatically performs speed change control for a plurality of ranges, i.e., a reverse drive range, a neutral range and a forward drive range (D range, Second, First, etc.) which are individually selected by the operation of the manual valve operated in correspondence to the manipulation of the shift lever (generally, this automatic control is effective only in the forward drive range).
Recently, another type of automatic transmission has been introduced. This automatic transmission does not use a manual valve for the selection of a range and controls the shift of speed change ratios in each range solely by electrical signals. Such transmissions are disclosed in Japanese Laid-Open Patent Publication Nos. H5 (1993 )-209683(A) and H5 (1993) -215228 ( A). Each automatic transmission disclosed there comprises a plurality of solenoid valves to control the actuation of shift control valves. The selection of the ranges, i.e., the forward drive range, the neutral range, and the reverse drive range, as well as the selection of the speed change ratios in the forward drive range are performed in correspondence to the command signals (electrical signals ) which actuate these solenoid valves.
To secure reliability of such speed change control systems, it is important to take measures to prevent electrical and mechanical failures which may cause, for example, a valve locking. Therefore, the systems disclosed in the above mentioned patent publications also include preventive measures against possible failures. As the selection of the ranges and the selection of the speed change ratios are carried out by means of command signals which actuate the solenoid valves, it is necessary to distinguish accurately the patterns of the command signals because they may resemble with one another. For example, a pattern of signal combination for the forward drive range may resemble a pattern of signal combination for the reverse drive range. In such a case, it is necessary to provide a backup circuit to assist the differentiation by the system of these two patterns, but the provision of such a circuit may be a disadvantage in view of production cost.
To avoid such problems, the pattern of signal combination for the reverse drive range is made significantly different from the pattern of signal combination for the forward drive range. Specifically, a plurality of solenoid valves are operated in different actuation logics. In this way, a prevention against selection of an inappropriate range is effected. In this system, even if, for example, a solenoid valve or a shift valve may actuate differently from an intended pattern by any reason, drive ranges and speed change ratios are selected appropriately. However, such a system requires a number of solenoid valves, thus increasing the production cost and size of the system.
It is an object of the present invention to provide a control system for an automatic transmission which selects drive ranges and speed change ratios solely by means of electrical signals without use of a manual valve.
Another object of the present invention is to provide a control system for an automatic transmission, which system is capable of selecting appropriate drive ranges even in an event of electrical or mechanical failure.
In order to achieve these objectives, the present invention provides a control system for an automatic transmission which comprises a forward/reverse hydraulic servomechanism (for example, the forward/reverse selection hydraulic servomechanism 70, which actuates the dog clutch 16, described as a preferred embodiment in the following section) and a plurality of hydraulic clutches (for example, the LOW clutch 11, the SECOND speed clutch 12, the THIRD speed clutch 13, the FOURTH speed clutch 14 and the FIFTH speed clutch 15 of the following embodiment). The forward/reverse hydraulic servomechanism is to switch and select either a plurality of forward drive power transmission paths or a reverse drive power transmission path. The hydraulic clutches are to selectively establish forward drive speed change ratios, and each hydraulic clutch is provided for a corresponding one of the forward drive power transmission paths. To control this automatic transmission, the control system according to the present invention comprises a plurality of solenoid valves (for example, the firstxcx9cfifth on/off solenoid valves 81xcx9c85 and the firstxcx9cthird linear solenoid valves 86xcx9c88 of the following embodiment) to control the supply and drain of a line pressure, which is used to actuate a plurality of shift control valves (for example, the firstxcx9cfifth shift valves 60, 62, 64, 66 and 68 of the following embodiment) for controlling actuation of the forward/reverse hydraulic servomechanism and the hydraulic clutches. The control system further comprises forward drive selection detecting means (for example, the hydraulic switch 92 of the following embodiment) for detecting whether the forward drive power transmission paths are selected by the forward/reverse hydraulic servomechanism or not and comprises actuation command signal detecting means for detecting actuation command signals which are sent to the solenoid valves. In this arrangement, if the forward drive selection detecting means detects that the forward/reverse hydraulic servomechanism has selected the forward drive power transmission paths, and if the actuation command signal detecting means detects that actuation command signals are being sent to set a forward drive speed change ratio, then the solenoid valves are allowed to actuate in response to the actuation command signals.
With this construction, only when the forward drive selection detecting means detects that the forward/reverse hydraulic servomechanism has selected the forward drive power transmission paths, the solenoid valves will actuate in response to the actuation command signals. As a result, for example, even if wrong on/off signals are sent because of an electrical failure, or even if some solenoid valves malfunction because of a mechanical failure, which may otherwise have taken the transmission into a wrong or unintended range, there is no possibility of the transmission switching to another range because the forward/reverse hydraulic servomechanism remains at the selection of the forward drive power transmission paths. Moreover, even if a combination of signals which may resemble to the pattern of signals setting one of the driving modes of the forward range is used for establishing another range, there is no possibility of malfunctioning.
It is preferable that the forward drive selection detecting means comprise a hydraulic switch which detects a forward drive hydraulic pressure that actuates the forward/reverse hydraulic servomechanism to select the forward drive power transmission paths. In addition, it is preferable that the control system further comprise an oil passage which leads the forward drive hydraulic pressure through the shift control valves (for example, the fourth shift valve 66 of the following embodiment) to the hydraulic switch when a forward drive speed change ratio is established. With this arrangement, for example, when the forward drive range is to be established, if a shift control valve does not shift to the side assigned for setting a forward drive speed change ratio because of an electrical failure, and even if the forward drive hydraulic pressure is being supplied to the forward/reverse hydraulic servomechanism, setting it to select the forward drive power transmission paths, the forward drive hydraulic pressure is never supplied to the hydraulic switch in this condition. As a result, the operation to establish the forward range is averted in this case. In this way, the control system according to the present invention provides additional safety.
Furthermore, it is preferable that the actuation command signals comprise different combinations of on/off signals to turn on or off the corresponding solenoid valves, each combination being specific to set a corresponding forward drive speed change ratio or to control the transition from one speed change ratio to a next speed change ratio. When the control system changes the mode of the transmission between neighboring speed change ratios, at first, one on/off signal in the combination of on/off signals being used for setting the present speed change ratio is switched to form another combination of on/off signals which combination comprises actuation command signals to control the transition from the present speed change ratio to a next speed change ratio. Then, another on/off signal in the combination of on/off signals being used for controlling the present transition is switched to form yet another combination of on/off signals which combination comprises actuation command signals to set the next speed change ratio.
For the speed change control between the neighboring speed change ratios, this control system changes the mode of the transmission from the present speed change ratio to a corresponding transitional control only by changing the command signal sent to one solenoid valve, and then, from this transitional control to a next speed change ratio, again only by changing the command signal sent to another solenoid valve. In this way, the signals sent to a plurality of solenoid valves are never changed in plurality, so the speed change control is performed simply with much stability.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.